In recent years, there continues to be dramatic density increases in integrated circuit technology for semiconductor chips. For example, the minimum feature size of lithography, such as the size of MOSFETs, has presently been reduced to one micrometer and below. Many applications implemented on modern semiconductor integrated circuit (IC) chips require accurate operation, which becomes increasingly difficult to provide as chip density continues to increase.
Among the specific problems that can occur in electrical circuits, particularly semiconductor circuitry, is leakage current through the transistors. As is well known, as the threshold voltage in a metal-oxide-semiconductor field-effect transistor (MOSFET) decreases, leakage current typically increases. Among the more common causes for a decrease in desired threshold voltage is an increase in temperature or process variations during the manufacturing process for the MOSFET that resulted in a lower threshold voltage than intended or desired.
To combat such leakage current, a back-bias voltage is typically applied to the well of the MOSFET to help increase the potential difference across the MOSFET, and therefore increase the threshold voltage of the device. Early approaches implemented a string of diodes coupled to the MOSFETs to decrease the leakage current, but this approach suffered several disadvantages. A primary disadvantage was a reduced back-bias voltage as the threshold voltage continued to decrease, which in the end resulted in more leakage current. Another popular conventional approach is to employ a back-bias voltage generator along with a back-bias voltage regulator circuit. Unfortunately, even this approach suffers from disadvantages, particularly the use of a fixed reference voltage on which the regulation of the back-bias voltage is based. Such a fixed reference voltage, and thus a fixed back-bias voltage, prevents the circuit from tracking any changes in the MOSFETs, even those that directly affect the amount of leakage current such as temperature and process variations. Accordingly, what is needed is a back-bias voltage regulator circuit that does not suffer from these disadvantages.